ABSTRACT
This study explores the impact of thermal annealing gradients on the physical properties and structural evolution of cadmium sulphide (CdS) nanospheres capped with ammonium nitrate as a modifier, which were fabricated through precipitation and subsequent annealing within 160-480 °C temperature range. The properties were characterized using X-ray diffraction (XRD), ultraviolet-visible (UV-Vis), Fourier transform infrared (FTIR), scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) techniques The XRD results show that the present CdS exhibits superior crystallinity compared to pure CdS without capping, transitions from a cubic to a hexagonal phase structure, and increases in crystallite size and crystallinity with increasing temperature. The FTIR spectra postulate that a vibrational band presence evidences ammonium nitrate capping on CdS, with another distinct band that represents CdS in the lower wavenumber region, both intensifying at elevated temperatures. The UV-Vis analysis reveals that CdS exhibits strong ultraviolet (UV) absorption suitable for effective photoreaction under UV light and has a broader band gap compared to bulk CdS. SEM images show an extensive distribution of homogeneous nanospheres over the surface, with increased growth in size when capped with ammonium nitrate and at higher temperatures. As validated by TGA and DSC results, CdS with a smaller crystallite size improves thermal stability and energy transfer, as evidenced by reduced weight loss and a lower endothermic temperature, respectively. Varying the annealing temperature with ammonium nitrate capping can improve the structural and physical properties of CdS, which are beneficial for varied applications such as optoelectronics, energy storage, and photocatalysts.
ABSTRACT
This is the first paper to report on the pH response to heterogeneous wurtzite/zinc blende phase transformation, optical tunability and thermal stability advancement of the CdS nanoparticles synthesized via co-precipitation, followed by subsequent thermal treatment at a desired annealing temperature of 320 °C, while the solution pH was varied during CdS synthesis by adjusting the ammonium salt concentration. The surface morphology, crystalline structure, functional groups, optical properties and thermal stability of CdS were characterized by scanning electron microscopy (SEM), X-ray diffractometer (XRD), Fourier-transform infrared spectroscopy (FTIR), UV-visible spectrophotometer, thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC), respectively. The results show that a dominant sharp band occurs in the FTIR spectra, which authenticates the presence of Cd-S bonds. XRD results reveal that as the pH declines, CdS in the initial cubic phase has gradually transformed into a heterogeneous phase with the coexistence of cubic and hexagonal structures. As observed from the SEM images, the CdS nanoparticles display a homogeneous, smooth and spherically shaped morphology. Optical absorption characterized by UV-visible spectrophotometry denotes that the band gap decreases proportionally with pH, which could be attributed to the formation of larger grain sizes from the aggregation of many small nanocrystallites. TGA and DSC analyses demonstrate an improvement in the thermal stability of CdS with increasing pH values. Consequently, the present findings dictate that pH tunability could be a valuable approach to procuring the desired properties for the respective applications of CdS in diverse fields.